1. Field of the Invention
The present invention relates to structural means for heating, that incorporate a block heater for use when bonding a chip to a lead frame using an adhesive, or for a heating roller for fusing a toner image in an electronic photocopying machine, or for dies into which a molten synthetic resin is injected for manufacturing a molding, etc., in a production process for semiconductors, for example. The invention is concerned more particularly with structural means for uniform heating that can heat an object evenly by uniform conduction of heat.
2. Description of the Related Art
When manufacturing semiconductors, for instance, there is a process in which a metal strip is fed in as a frame material, a plurality of lead-frame patterns are manufactured and arranged using progressive-feed dies, a predetermined length of the metal strip is heated, and a chip is bonded onto the mounting surface of the lead frame pattern using a metal adhesive. In the bonding process, it is required that the metal strip should be heated uniformly over the entire area in the longitudinal and lateral directions, and a block heater is widely used for heating the metal strip uniformly. In the process for bonding chips when manufacturing semiconductors, the metal strip is fed intermittently and heated by a block heater which is or is not in contact with the strip.
An example of a block heater consists of a main block unit with a rectangular parallelepiped shape using, a heat-resistant metal, and a heater is embedded inside the main block unit, as a heat source. And, a current is applied to the heater from outside to heat the main block unit totally, and in the case of bonding, for instance, an adhesive such as a metal paste is cured by the heat transferred from the main block unit.
In this type of bonding process, it is particularly important to heat the object, i.e., the total surface of the lead frame accurately and uniformly. More explicitly, when all or part of the surface of the main block unit is used as heat transfer surface for the lead frame, the heat will not be evenly transferred to the entire surface of the lead frame unless the temperature distribution on the whole heat transfer surface is uniform. If uneven heat transfer occurs, irregular curing of the metal paste may take place, and eventually, might result in distortion of the lead frame. In addition, such distortion may produce a crack in a chip, and as a result, the quality or performance of the product might be affected adversely.
As described above, it is very important to maintain a uniform distribution in the amount of transferred heat. However, with the block heater, a heater element is embedded in the block unit and sealed, so there is considerable difficulty in achieving a uniform temperature distribution on the heat transfer surface. Therefore, a somewhat uneven transmission of heat cannot be avoided completely. With a conventional block heater known in the prior art, there is an uneven distribution of transferred heat as shown in FIG. 16A and FIG. 16B.
Referring to FIG. 16A, when the main block unit 51 is shaped simply as a parallelepiped and the heater 52 is shaped as a rod with a circular cross section, for the purpose of simplifying the manufacturing process, the heat transmission surface 51a at the upper end surface of the main block unit 51 has the temperature distribution shown in FIG. 16B. That is because the main block unit 51 is heated by heat conduction from the heater 52 and dissipates heat from the heat transfer surface 51a. Therefore, near an axial end of the heater 52, the amount of transferred heat is smaller than at the center portion. Furthermore, because heat is also dissipated from the end surfaces of the block unit 51, the temperature of the block surface becomes lower towards both ends of the block unit 51.
The temperature distribution of such a block heater might be improved by modifying the shape of the block unit 51 appropriately, or by optimizing the arrangement or disposition of the heater 52, at least to some extent.
However, if the block unit 51 is shaped with extraordinary specification, the shape must be designed individually according to the size and shape of the object being heated, so a main block unit may not be put in common use. Also if the arrangement of the heater 52 is improved, the structure would be more complicated, therefore it might be impossible to restructure an existing block heater into another shape which will make the heat transmission uniform.
On the other hand, a conventional electronic photocopying machine is provided with a heat-fusing device in which a developed toner image is transferred to a paper sheet, and then the image is fused by melting the toner. With such a heat-fusing device, it is very important that the heat transfer from the heating roller to the paper sheet should be uniform, that is, the sheet should be heated with an even temperature distribution. Therefore, to obtain a uniform temperature distribution on the periphery of a heating roller, various effective means have been disclosed in the Japanese Unexamined Patent Publications Nos. 1-157083 and 3-267976, for example.
According to the inventions described in these publications, certain effects can be expected in producing a uniform temperature distribution in a heat-fusing roller. However, although the uniformity of the temperature distribution of the roller can be improved for heat-fusing a toner image, the roller cannot be readily applied to other fields of use.
In more detail, when fusing a toner image on a paper sheet, the periphery of a roller is not stressed so much by external loads and also a stress concentration thereto is rather small. Therefore, as shown in Japanese Unexamined Patent Publication No. 3-267976, the outer periphery can be composed of a cylinder made of a second metal with a high thermal conductivity and low mechanical strength. However, in the case of a printing hot roller or the like which is highly loaded on its periphery, with this method, it would be important to prevent flaws developing on the outer surface of the periphery. Consequently, it is not appropriate to apply a metallic material with a high thermal conductivity to the outer periphery.
Another method disclosed in Japanese Unexamined Patent Publication No. 1-157083 proposes to provide a resistance heating layer on the outer periphery of a cylinder with a high thermal conductivity, but because the resistance heating layer is exposed, this type of heater cannot cope with a large external load or stress concentrations.
Moreover, a conventional sintering vessel known in the prior art and used for alumina etc. in the field of, for instance, hot pressing, can produce a good sintered product by making the heat transfer to the material uniform. Therefore, with this type of heater, it is effective to use a material with a high thermal conductivity to establish an even distribution of temperature on the inner wall of the vessel. However, even if the aforementioned conventionally used structure is applied without modifications, problems still remain regarding durability such as its resistance to external loads and stress concentrations.
Conventionally, when manufacturing a plastic product using dies, a high-temperature, molten resin is injected into the dies and then cooled, and after curing, the molded product is separated from the dies and removed. At that time, it is preferred that the injected resin should be cooled uniformly. However, there is a tendency that the resin in contact with parts of the dies with thin wall thickness is cooled and cured earlier than the other portions. Consequently, it is preferred that the dies should be heated and cooled evenly.
However, even with these types of dies, there are large external loads as well as large thermal stresses on the inner surfaces of the dies, so the construction described in Japanese Unexamined Patent Publication No. 1-157083 is not sufficient, as in the case of the sintering vessel mentioned above.
As described above, when a conventional block heater is used in heat-printing rollers or dies, the prior art does not present means that can produce a uniform temperature distribution on the surface of the block and can keep the block stable when subjected to external loads, stress concentrations or thermal stresses.